Novel microfluidic devices are being developed for various applications, including drug delivery, rapid chemical synthesis, biological diagnostics and electronics cooling. The ability to actuate and control fluid in small amounts with high precision and flexibility is critical to the success of microfluidic operations. Conventional pressure-driven pumping methods are inadequate in accommodating these requirements mainly due to the large pressure head needed; moreover, the use of an external pump in a microfluidic system defeats the purpose of miniaturization. Alternative solutions have been sought and a variety of innovative micropumping concepts have been proposed in the literature. One particularly attractive scheme is to generate the required flow directly in the microfluidic devices by inducing strong electromechanical forces in the fluid through electrokinetic effects. Based on the origin of the electromechanical forces, electrokinetic micropumps can be classified as electrohydrodynamic (EHD), electroosmotic (EO), and AC electroosmotic (AC EO), among others. The common feature of these micropumps is to actuate the liquid via an induced body force directly exerted on the fluid element. Recently, more complex fluids, such as colloidal suspensions containing a second phase (vapor bubbles, solid/soft particles or immiscible liquid droplets) have received attention in microfluidics research and applications. Examples include separation/concentration of biological cells in micro-total-analysis systems (μTAS) and application of nanofluids in advanced cooling systems. Due to the presence of the second phase in the fluid, another important electrokinetic effect, dielectrophoresis (DEP), can be exploited to generate effective microfluidic pumping upon the application of an external electric field.
Dielectrophoresis is the motion of small particles in colloidal suspensions when exposed to non-uniform electric fields, arising from the interaction of the induced dipole on the particle with the applied field. Dielectrophoresis has been employed extensively as a powerful tool for manipulating particles in biological research, such as in separation, trapping, sorting and translation of cells, viruses, proteins and DNA. However, DEP research to date has focused on controlling the electromechanical response of the solid particles, while largely neglecting the hydrodynamic interactions between the particles and the surrounding fluid, i.e., the motion of the surrounding fluid induced by drag from the dielectrophoretic particle motion due to viscous effects. In spite of the advances in colloid science and electromechanics, a gap still persists in the application of advances in the science of particle dynamics and low Reynolds-number hydrodynamics to the DEP technique. This gap must be bridged to facilitate the implementation of DEP in a broader range of applications. In particular, the potential of traveling-wave DEP (twDEP) as an effective means for microfluidic flow actuation has not yet been explored.